In order to obtain the basic guideline for selecting materials used for smokestack exposed in a water dew point environment with SO2 gas (500, 2000ppm), corrosion tests of various kind of stainless steels were conducted by use of a laboratory made alternating dry-wet gas corrosion testing apparatus. Austenitic stainless steels, duplex stainless steels and superalloys showed very high corrosion resistance at a corrosion rate of less than 0.05g/(m2·h). While, low-alloy steel and ferritic stainless steels were less resistant against corrosion at corrosion rate of higher than 0.4g/(m2·h). In a water dew point environment with SO2 gas, the corrosion index (SO2/water dew point corrosion index), S.C.I=[Cr]+2.1[Mo]+1.2[Cu] was obtained. Stainless steels with S.C.I of higher than 20 were highly resistant to corrosion. Surface film formed on the austenitic stainless steels in the thickness range of hundreds to thousands Å were mainly consisted of oxides. It was observed that austenitic stainless steels became higher resistant to corrosion, the thinner the surface film was.
